1. Field of Invention
The invention relates to a fluid pressure actuated safety valve for allowing normal controlled flow and prohibiting abnormal uncontrolled back flow of fluid pressure through a drill string.
2. Description of the Prior Art
Safety float valves are used to prevent plugging of the drill bit nozzle by the cuttings during trips into the bore hole, to prevent back flow during connection of drill string components, and to eliminate the need for a mud pill when tripping out since the driller can "shake down" the mud in the drill pipe. Float valves are commercially available as a completely sealed or a controlled leakage type valve such as a flapper valve with a small hole therein. The controlled leakage type valve allows for slow control fill up of the drill pipe and enables accurate drill pipe pressure measurement to be taken during a kick or blowout. These float valves are sometimes used as drill string blow-out preventers or safety valves but are not generally reliable because of rapid damage and erosion by the drilling fluid.
Drop in check type safety valves which permit flow of fluid in one direction only are also available and used to prevent back-flow and blow outs. Usually the check valve tool is stored and installed in the drill string only when a kick occurs and therefore are not subject to deterioration or erosion and abuse by the drilling fluid. However, there is the danger during high stress, such as a kick, that the drilling personnel might install the tool containing the check valve upside-down or improperly to resist existing back flow conditions. Also, during an emergency and confusion personnel may not realize that other tools previously installed in the drill string may need to be removed before the check valve tool could be installed in the drill string. Another drill string safety float valve disclosed in Canadian Pat. No. 1,058,477 has, unlike the applicant's, means for delaying the closing of the valve and prohibiting back flow through the drill pipe of any abnormally pressurized fluids.
The applicant's three position downhole safety float valve provides operational and safety advantages over the conventional float and drop in check valves. The valve is installed and remains in the drill string to automatically shut in the well if proper action is not taken by personnel at the surface during a kick. In its normal partially open position the valve permits controlled fill up and drainage of the drill pipe while tripping into and out of the bore hole. When displaced by fluid under pump pressure to its fully opened position the relationship of the valve seat to the displaced valve stem is such as to minimize erosion thereof by the drilling mud during normal drilling operations. In the event of uncontrolled back flow up the drill pipe in excess of the present amount of differential fluid pressure applied thereto, the valve shifts rapidly to its closed position where it prohibits the up flow of fluid therethrough.